Stochastic model for photoinduced anisotropy

We present a stochastic model for the kinetics of photoinduced anisotropy in a sample of molecular chromophores that may undergo photoisomerization. It is assumed that the chromo‐phores do not interact among them, but are embedded in a medium that slows down the rotational diffusion. The model makes use of data about the photoinduced reorientation of the single chromophore, its photoisomerization and its rotational diffusion, that are made available by molecular dynamics simulations. For the first time such molecular scale processes are computationally connected to the development of anisotropy in a large sample and on a long time scale. A test on azobenzene shows the potentiality of the method and the interplay between photoinduced anisotropy and photoisomerization. © 2012 Wiley Periodicals, Inc.     

Absorption spectra of photoinduced E isomers of aurones,thioindogenides, and selenoindogenides

Theoretical and Experimental Chemistry -     

Flow injection determination of vitamin K3 by a photoinduced chemiluminescent reaction

The determination of vitamin K3 using the coupling between a photochemical reaction and a chemiluminescent reaction in a flow system is described. The method is based on the photooxidation of ethanol sensitized by vitamin K3 to yield hydrogen peroxide, which is monitored through the chemiluminescent reaction with luminol catalysed by hematin. The new approach allows the determination of vitamin K3 in a wide concentration range (1 x 10(-7)-5 x 10(-4) mol l-1) with a throughput of 30 samples h-1. The applicability of the method was demonstrated by the determination of vitamin K3 in pharmaceutical preparations.     

Electron-transfer mechanism in radical-scavenging reactions by a vitamin E model in a protic medium

The scavenging reaction of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH.) or galvinoxyl radical (GO.) by a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), was significantly accelerated by the presence of Mg(ClO4)2 in de-aerated methanol (MeOH). Such an acceleration indicates that the radical-scavenging reaction of 1H in MeOH proceeds via an electron transfer from 1H to the radical, followed by a proton transfer, rather than the one-step hydrogen atom transfer which has been observed in acetonitrile (MeCN). A significant negative shift of the one-electron oxidation potential of 1H in MeOH (0.63 V vs. SCE), due to strong solvation as compared to that in MeCN (0.97 V vs. SCE), may result in change of the radical-scavenging mechanisms between protic and aprotic media.     

Investigation of photoinduced electron transfer in model system of vitamin E-duroquinone by time-dependent density functional theory

Photoinduced electron transfer of the model system composed of vitamin E and duroquinone has been investigated using time-dependent density functional theory. Calculations for the excited states tell that the photoexcitation of the model system can directly yield the charge transfer states in which the vitamin E moiety is positively charged but the duroquinone moiety is negatively charged. Our theoretical investigations indicate that the second charge transfer state of the model system can also be produced through the decay of higher locally excited state S(4). Since S(4) state in the model system corresponds to S(1) state of the isolated duroquinone used as a model for peroxyl radical, and S(2) state has the character of electron transfer from the tertiary amine group of the vitamin E moiety to the duroquinone moiety, the decay from S(4) to S(2) corresponds to the dynamic process following the photoexcitation of the duroquinone moiety of the model system, i.e., the initial stage of antioxidant reaction of vitamin E. Calculations of the kinetic parameters for the electron transfer have been carried out in the framework of the Marcus-Jortner-Levich formalism. Our calculations confirm that the electron transfer from S(4) to S(2) possesses the character of the inverted regime and the barrier is negligibly small.     

Electrostatic interactions between model mitochondrial membranes

Lipids are very diverse in both their respective structures and functions; and cells exquisitely control membrane composition. One intriguing issue is the specific role of lipids in modulating the physical properties of membranes. Cardiolipin (CL) is a unique four-tailed, doubly negatively charged lipid found predominately within the inner mitochondrial membrane, and is thought to be influential in determining the inner mitochondrial membrane potential and permeability. To determine the role of cardiolipin in modulating the charge properties of membranes, this study investigated the electrostatic interactions between mixed cardiolipin and phosphatidylcholine bilayers as a function of cardiolipin concentration. For physiologically relevant concentrations of cardiolipin, the surface charge density of the membrane was found to increase linearly with increasing concentration of cardiolipin. However, only a fraction of the cardiolipin molecules predicted to carry a charge from pK-values were ionized. Clearly environmental factors, beyond that of pH, play a role in determining the charge of bilayers containing cardiolipin.     

A mathematical model for the interactions between non-identical rough spheres, liquid bridge and liquid vapor

Investigation of the interactions between the skeleton of an unsaturated particulate material and the contained liquid involves the general interaction model consisting of a liquid bridge in contact with two rigid smooth spherical particles of unequal size and dissimilar material, at a separation determined by their actual surface roughness, and surrounded by a gas with a vapor pressure at equilibrium with the liquid. The liquid retention and capillary force of the system are related to the capillary suction, liquid-solid contact angles, filling angles, roughness of the particles, and the ratio of particle radii in normalized terms by assuming a circular arc for the shape of the liquid profile. The normalized suction is also related to the corresponding relative humidity of the pore air. The calculated equilibrium relations are shown to possess non-uniqueness, which is interpreted in terms of mechanical properties of unsaturated particulate materials. The model is able to provide new insights into the behavior of an unsaturated particulate material.     

Explorations of the nature of the coupling interactions between vitamin C and methylglyoxal: a DFT study

As the first step toward understanding the augment role of vitamin C (Vc) for the anticancer effect of methylglyoxal (MG), the nature of the coupling interactions between Vc and MG has been systematically investigated at the B3LYP/6-311++G** level of theory in combination with the atoms in molecules (AIM) theory, natural bond orbital (NBO) method, and energy decomposition analysis (EDA). The possible stable complexes have been located on their potential energy surface (PES). Most of them are characterized by one or two intermolecular H-bonds with the binding energies varying from −11.1 to −2.0 kcal/mol. AIM analyses suggest that all the intermolecular H-bonds have been predominated by the electrostatic interaction. A good linear correlation between the intermolecular H-bond distance and the electron density as well as its Laplacian at the bond critical point of the intermolecular H-bond has been observed. Depending on the selected coupling modes between Vc and MG, the origin of the blue-shifts of the stretching vibrational frequencies of different C–H bonds has been elucidated. Additionally, the inherent reason for the positive role of Vc in the anticancer process for MG has been verified through the investigation of the one-electron oxidation behaviors of the most stable complex.     

Molecular complex formation between gadolinium(III) tetraphenylporphyrin and vitamin E

Summary The complex formation between Gd(III) tetraphenylporphyrin or free-base tetraphenylporphyrin and vitamin E have been studied by spectrophotometric titration in chloroform and cyclohexane solutions. It has been shown that tetraphenylporphyrin and its gadolinium complex form 1:1 molecular complexes with vitamin E. The absorption spectra of titrated porphyrins contain well defined isosbestic points. The equilibrium constants were found using curve fitting procedure. The observed interactions are stronger for metallated than for non-metallated porphyrin and in less polar than in polar solvents.

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Ausbildung von Molekülverbindungen zwischen Gadolinium(III)-tetraphenylporphyrin und Vitamin E

Zusammenfassung Die Ausbildung von Molekülverbindungen zwischen Gd(III)-Tetraphenylporphyrin oder der freien Base und Vitamin E wurden mittels spektrophotometrischer Titration in Cyclohexan und Chloroform untersucht. Es wird gezeigt, daß Tetraphenylporphyrin und sein Gadoliniumkomplex mit Vitamin E Molekülverbindungen äquimolarer Zusammensetzung bilden. Die Absorptionsspektren der titrierten Porphyrine zeigen gut definierte isosbestische Punkte. Die Gleichgewichtskonstanten wurden unter Zuhilfenahme einescurve-fitting-Algorithmus ermittelt. Die beobachteten Wechselwirkungen sind stärker für metalliertes als für nichtmetalliertes Porphyrin und in unpolaren als in polaren Lösungsmitteln.

     

Long-lived radical cations as model compounds for the reactive one-electron oxidation product of vitamin E

Heterocyclic compounds with structures similar to vitamin E, but without the hydroxyl hydrogen atom, were synthesized and their electrochemical behavior examined in acetonitrile solutions and as solids in aqueous solutions of varying pH by attaching the compounds to the surface of a glassy carbon electrode. Compound 1, containing a fully methylated aromatic ring was found to be the most long-lived following one-electron oxidation, with its radical cation (1+*) surviving in acidic aqueous solutions and able to be isolated as a salt, 1+*(SbF6-), when reacted with NOSbF6 in CH3CN. Electrochemical, UV-vis and FTIR experiments on 1+*, in addition to the results from theoretical calculations, indicated that the electrochemical, electronic and structural properties of 1+* are very similar to those of the radical cation of vitamin E.     

13C CP/MAS NMR studies of vitamin E model compounds

13C cross-polarization magic angle spinning (CP/MAS) NMR data for 2,2,5,7,8-pentamethylchroman-6-ol (2), 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox c) (3) and its acetate (4), 2-methoxy-2,2,5,7,8-pentamethylchroman-6-ol (5), 2-hydroxy-2,2,5,7,8-pentamethylchroman-6-ol (6) and 2,2,5,7,8-pentamethylchroman (7) are reported. A deshielding of 7.7 ppm for the carboxylic carbon was observed in solid Trolox due to formation of intermolecular hydrogen bonds within cyclic dimers. Such crystal packing permits effective cross-polarization and fast relaxation (short T1rho(H)). The impact of the proton concentration on the CP dynamics is reflected by the longer T(CP) and T1rhoH for Trolox-d2 (deuterated at mobile proton sites). The calculated GIAO RHF shielding constants are sensitive to intramolecular effects: rotation around the C-6-O bond (changes of sigma up to 8 ppm) and conformation at C-2.     

Solvent-controlled photoinduced electron transfer between porphyrin and carbon nanotubes

beta-Cyclodextrin (beta-CD)-modified multiwalled carbon nanotubes (MWCNTs) were successfully prepared by reaction of surface-bound carboxylic chloride groups of MWCNTs with aminoethyleneamino-deoxy-beta-CD (ENCD) and comprehensively characterized by FTIR, Raman, and X-ray photoelectron spectroscopy and thermogravimetric and differential thermal analysis. The beta-CD-modified MWCNTs (ENCD-MWCNTs) are highly water-soluble, with a solubility of ca. 9.0 mg x mL(-1). Furthermore, the photoinduced electron transfer (PET) process between tetrakis(4-carboxyphenyl)porphyrin (TCPP) and ENCD-MWCNTs was investigated by means of fluorescence, fluorescence decay, transient absorption spectroscopy, and cyclic voltammetry. Obvious quenching processes were observed upon addition of both MWCNT-COOH and ENCD-MWCNTs to the aqueous solutions of TCPP, indicating that the PET process between TCPP and carbon nanotubes takes place upon irradiation. When 1-adamantane acetic acid was added to the aqueous solutions of TCPP/MWCNT-COOH and TCPP/ENCD-MWCNTs, respectively, the former fluorescence remains, while the latter fluorescence recovers. On the contrary, the fluorescence intensity of TCPP in the DMF solution was hardly decreased upon addition of ENCD-MWCNTs, whereas its fluorescence was quenched in the presence of MWCNT-COOH. The observations indicate that the CD cavities play a vital role on the control of the PET process.     

Fluorescence-quenching phenomenon by photoinduced electron transfer between a fluorescent dye and a nucleotide base.

Fluorescently labeled oligonucleotide probes have been widely used in biotechnology, and fluorescence quenching by the interaction between the dyes and a nucleobase has been pointed out. This quenching causes big problem in analytical methods, but is useful in some other cases. Therefore, it is necessary to estimate the fluorescence quenching intensity under various conditions. We focused on the redox properties of some commercially available fluorescent dyes, and investigated dye-nucleotide interactions between a free dye and a nucleotide in aqueous solution by electrochemical and spectroscopic techniques. Our results suggested that the quenching was accompanied by photoinduced electron transfer between a thermodynamically quenchable excited dye and a specific base. Several kinds of fluorescent dyes labeled to the 5'-end of oligonucleotide C10T6 were prepared, and their quenching ratios compared upon hybridization with the complementary oligonucleotide A6G10. The quenching was completely reversible and their efficiencies depended on the attached fluorophore types. The fluorescence of 5-FAM, BODIPY FL or TAMRA-modified probe was strongly quenched by hybridization.     

DNA与Rt、VB2相互作用的电化学性质

芦丁(Rt)是存在于多种植物体中的一种多羟基黄酮化合物,是某些中草药的有效成分。本文在前文基础上着重对芦丁、VB2在玻碳电极上的电化学性质、二者与DNA相互作用的机理等方面的差异进行比较,这对维生素类药物与生物大分子相互作用的研究与应用具有重要意义。     

Modelling interactions between a PBB and fullerenes

Fullerenes have many uses including in medical and electronic nanodevices. High pressure liquid chromatography (HPLC) columns are generally used to extract a certain structure of fullerne from a mixture of them. In this paper, we investigate the interactions between various types of fullerenes and a station phase in HPLC known as pentabromobenzyl (PBB). The Lennard-Jones potential and a continuum approach are employed to determine the van der Waals energy of these interactions within the HPLC columns. The equilibrium configurations for any given distance between a fullerene and the centre of a PBB are obtained. Results of this study may assist the design of a chromatography column for fullerene separation.     

A photoactivated artificial muscle model unit: reversible, photoinduced sliding of nanosheets

A novel photoactivated artificial muscle model unit is reported. Here we show that organic/inorganic hybrid nanosheets reversibly slide horizontally on a giant scale and the interlayer spaces in the layered hybrid structure shrink and expand vertically by photoirradiation. The sliding movement of the system on a giant scale is the first example of an artificial muscle model unit having much similarity with that in natural muscle fibrils. In particular, our layered hybrid molecular system exhibits a macroscopic morphological change on a giant scale (~1500 nm) relative to the molecular size of ~1 nm by means of a reversible sliding mechanism.     

Optically active diamide as a model for studying interactions between mineral salts and nylon type polyamides in methanol solutions

N,N′-Dicaproyl (–)1,2-diaminopropane (I) was used as a convenient model for the study of the optical activity of a nylon type polyamide: polysebacamide (–)1,2-diaminopropane (II). ORD of I was measured in different solvents. A peculiar behavior is observed in methanol in the presence of mineral salts. The influence of 0.1M potassium salts (Cl^?, Br^?, SCN^?, NO₃^?, SO₄^?2) and 0.1M alkaline chlorides and alkaline earth chloride hexahydrates on the optical activity of I in methanol are described. Alkaline salts and MgCl₂ give approximately the same effect: there is a decrease of the rotations without change of sign. SrCl₂ and CaCl₂ shift ORD curves towards the negative rotations, the last one giving complete inversion. This inversion is directly related to the CaCl₂ concentration and is attributed to adduct formation between amide groups and salt. Assuming that the different species are at equilibrium, an apparent equilibrium constant is obtained from the optical rotations for a complex of one mole of CaCl₂ with one mole of I. Results are used to discuss the complex ORD of poly(?) 1,2-diaminopropane sebacamide in methanol saturated with CaCl₂.     

Electroactive mixed self-assembled monolayers: Lateral interactions model updated to interactions between redox and non-redox species

The lateral interactions model, dedicated to random and non-random distributed electroactive species on redox responsive self-assembled monolayers (SAM), was extended to interactions between redox and non-redox species. This approach supports an unusual result achieved in the field of electrochemical transduction without covalent links between redox and complexant units in mixed SAM.